Many designs for vehicle seat belt retractors having a belt-winding mechanism that applies a relatively high rotational force to the belt reel when the belt is undone and a relatively low rotational force to the reel when the belt is connected to the buckle have been proposed in recent years. These designs are intended to overcome the problem of imparting a sufficient rotational force to the reel to wind the belt into it without pulling the belt against the vehicle occupant when it is buckled up for use so firmly as to be bothersome or even uncomfortable. These retractors usually have two winding springs coupled between the reel and the frame and a mechanism for deactivating one of the springs when the belt is done up to the buckle.
In U.S. Pat. No. 4,809,925 (Mar. 7, 1989) for "Seat Belt Retractor" (the subject matter of which was published in counterpart U.K. Patent Application GB No. 2 195 232, Apr. 7, 1988 and German Application DE OS No. 37 32 465, May 5, 1988) the present inventor has described and shown a seat belt retractor having a winding mechanism comprising a main winding spring coupled between the reel and frame, a clutch wheel rotatably carried by the reel shaft, and an auxiliary winding spring coupled between the frame and the clutch wheel. A lost-motion mechanism interposed between the clutch wheel and the reel shaft allows a predetermined amount of rotation of the reel shaft relative to the clutch wheel but couples the reel shaft to the clutch wheel at one end of the lost motion so that the forces of the two springs act in parallel to wind the belt onto the reel. The lost-motion mechanism allows the slack in the belt to be taken up when the auxiliary spring is deactivated and allows the belt to be pulled out when the occupant leans forward. The clutch wheel is locked against rotation in the belt-winding direction by a clutch-engaging mechanism composed of a lever and pawl operated by an electromagnetic solenoid that is energized when a switch in the buckle is closed upon insertion of the buckle tongue on the belt into the buckle.
The retractor described above has been found to perform entirely satisfactorily and reliably. On the other hand, it has been observed that the mechanical lost-motion mechanism, which consists of gears and a locking pawl, is noisy, and the noise it makes, especially when the belt is undone and the lost motion is taken up by winding of the auxiliary spring, can be disconcerting to the vehicle occupant. The gear noise and the noise of the impact of the pawl with the gear wheel abutment are, accordingly, undesirable.
French published patent application No. 2,478,471 (published Sept. 25, 1981) describes and shows a seat belt retractor having a tension-reduction mechanism in which a series of clutch discs act between the reel shaft and a clutch wheel to provide a lost-motion coupling between them. As many as five collisions may occur as the lost-motion is taken up when the tension-relief mechanism is deactivated, and bothersome noise can be expected.